The present invention pertains to a lift truck, preferably a shaft-steered traveling lift truck, comprising a drive unit pre-tensioned by spring loading in the vertical direction, which drive unit is hinged, adjustably in height, by means of a linkage parallelogram suspension on a chassis part of the lift truck which cannot be raised, and whose ground contact pressure can be varied depending on the loading force, and a lifting cylinder for the low lift (basic lift) of chassis parts that can be raised.
West German Patent No. DE-PS 31,06,027 discloses a lift truck with a wheel hub drive, whose height can be adjusted and in which the pre-tensioning of at least one spring of a spring-mounted wheel hub drive is reduced in order to reduce the tilting moment when the lifting height and/or the loading increase, as a result of which the ground contact pressure is highest and steering is consequently most difficult in the nonloaded state. Because the tendency of the vehicle to tilt is to be primarily counteracted with this device, its use is limited mainly to high-lift trucks. There is usually no risk of tilting in the case of low-lift trucks, but optimization of the driving, steering, and braking forces is desirable, instead. The optimal braking force, in particular, might be questionable because of the continuous decrease in the ground contact with increasing load. Aside from this, this device is more susceptible to trouble and requires a large amount of maintenance because of the use of complicated hydraulic means, and its application is limited to lift trucks of the design equipped with wheel hub drives.
In another shaft-steered material handling device disclosed in EP- A0,150,830, a drive wheel arranged between two lateral support wheels is pressed against the ground to ensure ground contact by spring pressure means that can be pre-tensioned; the basic setting of the drive wheel pressure should be as low as possible, and it is increased via a sophisticated pre-tensioning device with a plurality of electrohydraulic setting elements as a function of a defined operating parameter, preferably a braking initiation signal. The inclination of the surface, the moment of load of the drive motor, the nature of the ground, or an arbitrary signal that can be generated manually can instead be used as alternative operating parameters. This device has the disadvantage that the constantly changing load states of the material handling device are not taken sufficiently into account. Even though the shaft almost always remains readily movable by the operator, the acceptability of making the stopping distance that must be maintained under any circumstances dependent upon the function of a plurality of electrohydraulic setting elements, e.g., switches, valves, hydraulic storage containers, and hydraulic cylinders, is highly questionable for safety reasons, because the drive wheel pressure on the surface, which pressure is needed to maintain the stopping distance, is abolished in the case of failure of one of the setting elements, and the drive wheel may slip due to a so-called sliding friction, as a result of which there is a risk that the stopping distance will increase in an unacceptable manner.
According to a device for increasing the braking force in industrial material handling trucks, which is described in EP-A0,209,502, the significance of the dependence of the braking process on the differences in the load states of the material handling truck has been recognized. Based on this, the increase in the load of the material handling truck is taken into account there in a meaningful manner when increasing the drive wheel pressure by increasing the pre-tensioning of a spring of an elastically suspended adjusting device in a load-dependent manner in order to thus increase the drive wheel pressure. The design and the function of this device appear to be relatively complicated, which is also associated with limited reliability in operation as a consequence of the fact that the susceptibility to trouble due to malfunction or failure of a hydraulic setting member cannot be ruled out, besides the considerable production costs.